1. Technical Field
The present invention relates to cutting tools and articles having high wear resistance, which are prepared from silicon nitride-based ceramic systems, and more particularly, to silicon nitride-based ceramic systems having a relatively high proportion of alpha prime SiAlON, exhibiting relatively high hardness, toughness, and strength and being prepared from alpha silicon nitride starting materials.
2. Discussion
Silicon nitride materials currently occupy an important and growing portion of the market for materials for many commercial applications. It has become an increasingly important material, for instance, in the cutting tool and bearing industries, and continues to receive attention in the automotive component market. As compared with conventional materials, such as carbide-based materials or steel materials (e.g., M-50), silicon nitride generally offers the potential advantages of relatively high heat resistance and chemical stability, relatively low density, good mechanical properties such as hardness and toughness, and good electrical insulation characteristics. To illustrate the advantages, in the context of the cutting tool industry, these properties can combine in whole or in part to allow operations to proceed at higher speeds and temperatures, with resulting potential cost savings. The potential for the above properties and others also makes silicon nitride an attractive material (and its uses are believed to be under exploration) for other applications, such as without limitation, extrusion dies and automotive components (e.g., without limitation, turbocharger components, swirl chambers, engine valve or valve train components, etc.).
As persons skilled in the art are aware, there are two widely recognized groups of silicon nitride (Si.sub.3 N.sub.4) ceramics. One group is referred to as alpha silicon nitride (".alpha.-Si.sub.3 N.sub.4 "). The other group is referred to as beta silicon nitride (".beta.-Si.sub.3 N.sub.4 "). Beta silicon nitride has been the material of choice for cutting tools. Solid solutions of silicon nitride, in which some of the Si--N bonds are replaced by Al--O bonds and which often include other metallic elements, are known as SiAlON materials.
Further details of the characteristics of these groups of nitrides, and their solid solutions in SiAlON systems can be found in commonly owned co-pending application Ser. No. 08/753,359, and PCT/US 97/21263, hereby expressly incorporated by reference. Until the inventions that are the subject matter of that patent application Ser. No. 08/753,359, and this application, it was previously not believed that the combination of properties were obtainable wherein the resulting material comprised a relatively high portion of alpha-prime SiAlON and exhibited a hardness of at least 19 GPa and indentation toughness of at least about 5 MPa.cndot.m1/2.